The method of this invention is employed to reform panels of fibrous material such as fiberboard or hardboard, into curved self-supporting structures such as automotive headliners. Fiberboard or hardboard panels usually include resin binders which impart a substantial resistance to bending so that the sheet has a tendency to tear or crease when deformed or re-formed into a compound curvature. Moreover, the fiberboard panels tend to return to their original flat shape after being re-formed unless the fibers are actually shifted during the re-forming process.
The method includes applying water to a sheet of woodlike fibrous material, such as panels of ordinary fiberboard or hardboard which may or may not be perforated. The water is applied at an elevated temperature to the sheet in an amount sufficient to render the sheet ductile enough to permit forming without tearing or blowing and thereafter the water-treated sheet is formed to the desired curvature in a die under pressure. A water temperature between about 180.degree. F. and about 212.degree. F. is preferred in order to break down or soften the resin binders which permeate the body of the fiberboard or hardboard material. Temperatures as low as 120.degree. F. are sufficient for the purpose, however.
The panel need not have perforations. However, the presence of perforations to some extent will serve to weaken the structural makeup of the panel and thereby aid in permanent re-forming. Such perforations also provide voids into which fibers may be shifted during re-forming and furthermore provide a means for entry and escape of the water into and out of the body of the sheet during the re-forming process.
The water reduces the rigidity of the resin binders, whether thermoplastic or thermosetting, and moistens or lubricates and softens the structure of the fibrous material itself, but does not saturate or soak the panel material to an extent that would cause blowing when the panel is re-formed in the hot press. It is desirable that the panel be sufficiently moistened so as to be rendered ductile to permit forming without tearing or creasing, and yet not overtreated with water to the extent of soaking the internal structure and making it likely to blow when being formed. By "blowing" is meant the rupture of the panel from internal pressure caused by an excess of steam generated by too much water in the body of the panel when heated in the press.
The panel is preferably formed between heated dies under a pressure sufficient to actually cause material flow of the fibers in the areas of curvature. The re-forming takes place without tearing or blowing. When the panel is removed from the dies, most of the moisture has been driven out and the resin binders return to a rigid or semi-rigid condition to retain the re-formed shape.
After the panel has been water treated and re-formed in a heated press, a decorative and/or sound-absorbing layer may be applied thereto. This is accomplished by placing the re-formed panel upon a contoured support, applying adhesive to the upper surface of the panel, and laying the sound-absorbing layer over the adhesive coated sheet. The layer is adhered to the sheet preferably by draping a cover over the support to form an air seal and applying differential pressure, in this instance vacuum through the support to draw the cover toward the support and against the layer to adhere the layer to the sheet.
The prior art teaches the use of steam to render the panel ductile before placing it between heated dies. Thus in accordance with the prior art the panel is initially placed in a steam chest or chamber. However, in a steam chest, drops of water form and fall on the panel producing concentrated circles of excessive moisture which show up on the finished panel after it has been die formed and dried. The circles of excessive moisture are believed to form high spots by causing swelling of the panel material. The panel high spots tend to become over-compressed in the die and produce slight dimples which show through the sound-absorbing or decorative layer adhered to the panel. Such dimples or show-through marks are undesirable because they produce an unsightly appearance. Wet spots in the board due to drips from the steam chest also tend to show up in the finished panel through a painted surface in instances where the board is painted rather than laminated by adhesive to a sound-absorbing or decorative layer.
Panels treated with water at elevated temperatures rather than in a steam chest prior to die forming have been found to be free of the objection of water marks or wet spots.